This invention relates to current amplifiers and in particular to electrometer amplifiers, ie amplifiers for measuring very small DC currents such as those from ionisation chambers, eg 10.sup.-10 -10.sup.-15 A.
As is known, such amplifiers usually require quite complex initial zero-setting procedures, and have subsequent drift problems. As regards zero-setting, the procedure may involve first short-circuiting the input current to ground to prevent charge build-up on the input insulator, and then in turn zero-setting for voltage by adjusting the amplifier operating conditions and zero-setting for bias current by adjusting a compensating current from an external source. Finally the input current is reconnected for the measurement. This procedure must be repeated periodically if the amplifier zero drifts with time, eg due to temperature changes, as is normal.
Arrangements for alleviating these problems are known in which the input current is automatically switched periodically between two identical amplifier channels. During the period the current is switched to each channel there is zero input to the other channel, and a zero-correction circuit associated with each channel automatically restores the zero condition in that other channel ready for the input current to be re-connected thereto. The channel outputs are switched in synchronism with the inputs. However, in such existing arrangements each zero-correction circuit acts only to adjust the amplifier operating conditions and makes no correction for change in bias current or for other unwanted changes.